1. Field of the Invention
This invention relates generally to a millimeter-wave imaging system and, more particularly, to a millimeter-wave imaging system including an array of ferroelectric elements which sense radiation as a result of the pyroelectric effect.
2. Discussion of the Related Art
Present day imaging systems of the type used in connection with aircraft, including commercial, private and military aircraft, that provide detailed images of a scene are well known in the art. These imaging systems provide a mechanism so as to enable aircraft operators to avoid collisions and detect runways in low visibility conditions, such as at night and in foul weather. Such imaging systems are not limited to aircraft environments, however, but are also applicable in connection with other systems requiring imaging, such as surveillance systems and mapping systems.
Original imaging systems of this type made use of visible light detection by flying spot scanners, well known to those skilled in the art. However, visible light imaging had obvious limitations in low light situations and poor weather conditions because of the shortage of light and the limits of visible light to be transmitted through certain atmospheric conditions such as fog, haze, rain, dust, smoke, etc.
Radar imaging offered a next generation of detection which was fairly effective for imaging a scene in low visibility and poor weather conditions. Radar imaging is an active imaging system in that a transmitter is used to emit a signal which is then reflected from objects in the scene and detected by receivers associated with the radar imaging system. However, radar imaging does not provide the level of resolution necessary for many situations, thus limiting detection of many objects in many conditions. Another drawback with radar imaging is that the need for a transmitter provides greater chance of detection in stealth situations.
More recent imaging systems make use of infrared radiation as the mechanism by which objects are detected. Infrared radiation is effective for detecting objects because of the ability of infrared radiation to penetrate atmospheric condition such as fog, clouds, haze and the like. Unlike radar imaging, infrared imaging is a passive imaging system in that sensors associated with the infrared imaging systems rely on infrared radiation emitted from the objects in the scene. Infrared imaging systems which incorporate sensors that make use of the pyroelectric effect of ferroelectric films are known in the art. See for example, E. H. Putley, "Thermal Detectors in Optical and Infrared Detectors," 2nd ed., Topics in Applied Physics, Vol. 19, Springer-Verlas, New York. The pyroelectric effect occurs as a result of radiant energy causing an increase in the temperature of a ferroelectric material, which in turn causes a charge build-up in the material that can be detected. An infrared imaging system of this type incorporates an imaging array having a series of ferroelectric elements that are positioned on an appropriate substrate so as to detect infrared radiation. Charge induced by temperature changes in the ferroelectric elements caused by the radiation is sensed by appropriate signal processing circuits which then generate an output signal that is displayed by a video system.
Passive millimeter-wave imaging has been demonstrated as being an effective mechanism for penetrating the atmospheric conditions mentioned above so as to provide landing assistance and surveillance capabilities for aircraft. In many cases, millimeter-wave radiation can penetrate these atmospheric conditions better than infrared radiation. One known passive millimeter wave imaging device is disclosed in Lam, Wayne et al., "Millimeter-Wave Imaging Using Preamplified Diode Detector", IEEE MICROWAVE AND GUIDED WAVE LETTERS, Vol. 2, No. 7, July 1992, pp. 276-277. Known passive millimeter-wave imaging systems have relied on mechanical scanning of pencil beam antennas to cover a field of view with one or a few millimeter-wave receivers. These types of antennas are similar to the flying spot scanners used in visible light imaging. However, these mechanical scanning antennas do not provide the best signal-to-noise ratio and image updating which could be realized. Additionally, picture elements (pixels) of focal plane arrays in the known millimeter-wave imaging systems incorporate antennas, such as Vivaldi antennas, for each pixel. Such antennas add to the cost and complexity of the imaging systems.
What is needed is an imaging system that is able to provide an image through most atmospheric conditions and that includes detectors capable of providing a heightened signal-to-noise ratio. It is therefore an object of the present invention to provide such an imaging system.